The invention relates to image forming apparatus, such as printers, facsimile machines, or copying machines, which form an image on a sheet by transferring a developer image to the sheet. The invention further relates to a sheet transport method for transporting a sheet fed from a sheet storage.
Hereinafter a recording medium such as a sheet of paper or OHP film is collectively referred to as a sheet. In image forming apparatus, a sheet is transported through a sheet transport path to an image forming section which is positioned at an upper part of the apparatus. The sheet transport path extends vertically upward from a sheet feeding section to a transfer position where a developer image is transferred to the sheet. The sheet feeding section provided at a lower part of the apparatus has a plurality of sheet feeding cassettes arranged in tiers.
In an image forming process performed by the image forming apparatus, sheets stored in a selected one of the sheet feeding cassettes are fed, one at a time, into the sheet transport path with a device such as a pick-up roller. A sheet as fed is held by a pair of registration rollers, and is then delivered to the transfer position in a timely manner such that a developer image is properly transferred onto the sheet.
As described above, recent image forming apparatus generally have a vertical, and therefore short, sheet transport path which is fit for high-speed image formation. Such a vertical sheet transport path is also effective in downsizing image forming apparatus.
Depending on image forming apparatus with such vertical sheet transport path, however, a distance that a sheet is transported along the sheet transport path from the sheet feeding cassette to the transfer position is shorter than a distance that a developer image is transported from a developer image forming position to the transfer position. Therefore, formation of a developer image is initiated at an earlier timing than feeding a sheet is initiated. Accordingly, a delay in feeding a sheet prevents an already formed developer image from being transferred onto the sheet. The problem has a serious effect on an image formation process, particularly in a multi-color image forming apparatus using a tandem-type intermediate transfer method.
Such multi-color image forming apparatus has a plurality of image stations for forming developer images of respective colors. The image stations are arranged in alignment with one another. The developer images are sequentially superimposed on one another on an intermediate transferring carrier at respective intermediate transfer positions. The developer images as superimposed are then delivered to a transfer position. Because of a long distance between the transfer position and the intermediate transfer position which is most distant from the transfer position, formation of a developer image at the most distant intermediate transfer position needs to be initiated long before feeding a sheet for the developer image to be transferred to is initiated.
A sheet feeding failure sometimes occurs with developer images already formed in some of the image stations. To such failure, there are conventional solutions such as cleaning off untransferable developer images or delaying initiation of image forming processes (i.e., extending interval periods between image forming processes). However, the former solution has problems, such as of developer waste or of a recovering container getting shortly filled up with recovered developer. The former solution thus results in an increase in maintenance cost and therefore is uneconomical. The latter solution causes a decrease in image forming speed, i.e., a reduced number of image forming processes performed per unit time.
As another solution to sheet feeding failure, Japanese Laid-open Patent Application No. 2003-206044 discloses an image forming apparatus which, in the event of a failure in feeding a sheet, modulates pressure that a separating roller applies to the sheet, so that the separating roller refeeds the same sheet with an increased force.
Although the separating roller has an increased feeding force, a sheet transport speed is not increased in the sheet refeeding operation. Thus, the sheet takes a longer time to reach the transfer position than it takes under a condition where no sheet feeding failure occurs. Consequently, the sheet is prevented from being tranported to the transfer position in time for arrival of a developer image at the transfer position. Therefore, the developer image should be cleaned off, and a new developer image should be formed in order to be transferred to the sheet.
It is a feature of the invention to offer an image forming apparatus, and a sheet transport method, which allow a sheet to be transported in a timely manner for a developer image transfer operation, thereby preventing an increase in maintenance cost and a decrease in image forming speed.